Graphene has garnered widespread interest for use in a number of applications due to its favorable mechanical and electronic properties. The electrical conductivity of graphene can be influenced by the amount and type of chemical functionalization on the graphene and the quantity of defects in the graphene basal plane. Although pristine graphene typically displays the highest electrical conductivity values, it can sometimes be desirable to tune the electrical conductivity and adjust the band gap. Tailoring of the band gap can be accomplished by introducing a plurality of defects (holes) within the graphene basal plane. The band gap can be influenced by both the size and number of holes present.
Perforated graphene has also been found to have filtering capabilities. Indeed, it has been found that graphene with appropriately sized apertures can remove sodium ions and chlorine ions from water. This filtering capability is also believed to be adaptable to gasses, particulates, solutes, molecules and hydrocarbons, or any other nano-sized constituent from a medium.
It is known that apertures may be made by selective oxidation, by which is meant exposure to an oxidizing agent for a selected period of time. It is believed that apertures can also be laser-drilled. As described in the publication Nano Lett. 2008, Vol. 8, no. 7, pg 1965-1970, the most straightforward perforation strategy is to treat the graphene film with dilute oxygen in argon at elevated temperature. As described therein, through apertures or holes in the 20 to 180 nm range were etched in graphene using 350 mTorr of oxygen in 1 atmosphere (atm) argon at 500° C. for 2 hours. The paper reasonably suggests that the number of holes is related to defects in the graphene sheet and the size of the holes is related to the residence time. This is believed to be the preferred method for making the desired perforations in graphene structures comprising a single sheet or multiple sheets. The structures may be graphene nanoplatelets and graphene nanoribbons. Thus, apertures in the desired range can be formed by shorter oxidation times. Another more involved method as described in Kim et al. “Fabrication and Characterization of Large Area, Semiconducting Nanoperforated Graphene Materials,” Nano Letters 2010 Vol. 10, No. 4, Mar. 1, 2010, pp 1125-1131 utilizes a self assembling polymer that creates a mask suitable for patterning using reactive ion etching. A P(S-blockMMA) block copolymer forms an array of PMMA columns that form vias for the RIE upon redeveloping. The pattern of holes is very dense. The number and size of holes is controlled by the molecular weight of the PMMA block and the weight fraction of the PMMA in the P(S-MMA). Either method has the potential to produce a perforated graphene sheet or sheets.
Currently, there are no methods for reliably introducing holes under about 2.5 nm in size to graphene. In the size range of about 2.5 nm to about 10 nm, current techniques for perforating graphene take many milliseconds per hole, and there is no ability to form multiple holes in a single operation. Above about 10 nm, multi-step but laborious lithography techniques can be used. None of these techniques are amenable to introducing holes over a wide surface area.
In view of the foregoing, simple techniques that allow a plurality of holes to be simultaneously introduced to a graphene sheet and the ability to adjust the hole size would be of considerable benefit in the art. The present invention satisfies this need and provides related advantages as well.